Hello john35,
This is an excellent question and I'm excited to hear that you're ready to get started in QuickBooks Enterprise 2020.
You're absolutely right. When you upgrade to a newer version of the program, you'll need a new license number. As a customer with a subscription, you would have received an email about this around the release date of the product. I recommend checking through your email folders to see if you can find it.
If you can't, don't worry! Just give Enterprise support a call at 1-877-772-9158 and let them know you misplaced your license number. An agent will be happy to assist you from Monday to Friday between 9 a.m. and 8 p.m. ET. Being a Desktop Enterprise user, your support costs are included in your subscription.
In case you haven't had a chance to take a look at it, here's our article about the 2020 release: What's new in QuickBooks Desktop 2020
Have a great weekend!